Help with determining proper chain for 18-speed, please!

The chain is completely rusted so I can't tell what type it is. The bike is 18-speed with a Shimano 200GS biopace-SG crank that says, "for narrow chains only" on it. The derailleurs are Shimano 200GS.

Aardvark cycles used to have a page, but they're gone, and even Sheldon Brown doesn't seem to have a page comparing SG, HG, IG, etc. I feel completely goofy asking, but is there a "guide to chains" on the web somewhere? Can someone tell me what chain would work with this one? Since he isn't going to ride this off of fireroads and is slim on cash, I am thinking about sending him to K-mart for the chain.